142 



ROBERT H. HUTCHISON 



least, to the question : Are the descendants of a single individual 

 less variable than a population selected at random? an affirma- 

 tive answer can be given. 



The effect of cultural variation on thermal resistance is not 

 appreciable in amount. There are several important changes 

 which take place in a hay infusion during the course of growth, 

 namely, changes in acidity, increase in the amount of alkali and 

 of inorganic substances (Peters '07) (Fine '12). Yet in all the 

 cultures whose history was followed out for any length of time, 

 there seemed to be no change in the thermal resistance of the 

 forms present. The figures given above for Pure line no. 5 illus- 

 trates this. In fact all the pure line cultures agree in this respect. 

 As further evidence of this point two experiments with a pure 

 race of Paramecium aurelia are recorded here. The first set of 

 figures were obtained when the culture was ten days old, straw 

 colored and acid in reaction. The second set of figures were 

 obtained when the culture was two months old, and of a dark 

 brown color: 



It appears therefore that the effect of the ordinary changes 

 which occur in the medium is not profound enough to change the 

 thermal resistance of the forms present. 



The influence of the acidity of the medium deserves special 

 mention, because the amount of acid present changes more than 

 any other known ingredient. In a hay infusion the acidity rises 

 rapidly to a maximum within the first few days and then gradu- 

 ally declines (Fine '12). Yet in several cultures which were 

 followed during two, three, and in one case four months of their 

 history, the thermal resistance of the animals was not perceptibly 

 altered. In a few experiments, where Paramecium caudatum 



